AD5425 查看數據表（PDF） - Analog Devices

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AD5425

8-Bit, High Bandwidth Multiplying DAC with Serial Interface

Analog Devices 

AD5425

It is important to note that VIN is limited to low voltage because

the switches in the DAC ladder no longer have the same source

drain drive voltage. As a result, the on resistance differs, which

degrades the linearity of the DAC.

VIN must also not go negative by more than 0.3 V, otherwise an

internal diode turns on, exceeding the maximum ratings of the

device. In this type of application, the full range of the DAC

multiplying capability is lost.

ADDING GAIN

In applications where the output voltage is required to be

greater than VIN, gain can be added with an additional external

amplifier or it can be achieved in a single stage. It is important

to take into consideration the effect of temperature coefficients

of the thin film resistors of the DAC. Simply placing a resistor in

series with the RFB resistor causes mismatches in the temp-

erature coefficients and results in larger gain temperature

coefficient errors. Instead, the circuit of Figure 34 is a recom-

mended method of increasing the gain of the circuit. R1, R2,

and R3 must all have similar temperature coefficients but do not

need to match the temperature coefficients of the DAC. This

approach is recommended in circuits where gains of greater

than 1 are required. Note that RFB ≫ R2//R3 and a gain error

percentage of 100 × (R2//R3)/RFB must be taken into consideration.

VDD

R1

VIN

VREF

VDD

GND

RFB

IOUT1

IOUT2

C1

A1

R3

VOUT

GAIN = R2 + R3

R2

R2

R1 = R2R3

NOTES:

R2 + R3

1. ADDITIONAL PINS OMITTED FOR CLARITY.

2. C1 PHASE COMPENSATION (1pF TO 2pF) MAY BE REQUIRED

IF A1 IS A HIGH SPEED AMPLIFIER.

Figure 34. Increasing the Gain of Current Output DAC

DACs USED AS A DIVIDER OR PROGRAMMABLE

GAIN ELEMENT

Current steering DACs are very flexible and lend themselves to

many different applications. If this type of DAC is connected as

the feedback element of an operational amplifier and RFB is used

as the input resistor as shown in Figure 35, then the output

voltage is inversely proportional to the digital input fraction, D.

For D = 1 − 2−n, the output voltage is

VOUT = −VIN/D = −VIN/(1 − 2−n)

As D is reduced, the output voltage increases. For small values of D,

it is important to ensure that the amplifier does not saturate and

that the required accuracy is met. For example, an 8-bit DAC

driven with the Binary Code 0x10 (00010000), that is, 16 decimal,

in the circuit of Figure 35, causes the output voltage to be 16 × VIN.

Data Sheet

However, if the DAC has a linearity specification of ±0.5 LSB,

then D can in fact have a weight anywhere in the range 15.5/256 to

16.5/256. Therefore, the possible output voltage is in the range

of 15.5 VIN to 16.5 VIN—an error of 3%, even though the DAC

itself has a maximum error of 0.2%.

VDD

VIN

RFB

IOUT1

VDD

GND

VREF

VOUT

NOTE:

1. ADDITIONAL PINS OMITTED FOR CLARITY.

Figure 35. Current Steering DAC Used as a Divider or

Programmable Gain Element

DAC leakage current is also a potential error source in divider

circuits. The leakage current must be counterbalanced by an

opposite current supplied from the operational amplifier

through the DAC. Since only a fraction, D, of the current into

the VREF terminal is routed to the IOUT1 terminal, the output

voltage has to change as follows:

Output Error Voltage Due to DAC Leakage = (Leakage × R)/D

where R is the DAC resistance at the VREF terminal. For a DAC

leakage current of 10 nA, R = 10 kΩ. With a gain (that is, 1/D)

of 16, the error voltage is 1.6 mV.

REFERENCE SELECTION

When selecting a reference for use with the AD5425 current

output DAC, pay attention to the reference output voltage

temperature coefficient specification. This parameter not only

affects the full-scale error, but can also affect the linearity (INL

and DNL) performance. The reference temperature coefficient

must be consistent with the system accuracy specifications. For

example, an 8-bit system required to hold the overall specification

to within 1 LSB over the temperature range 0°C to 50°C dictates

that the maximum system drift with temperature must be less

than 78 ppm/°C. A 12-bit system with the same temperature

range to overall specification within 2 LSB requires a maximum

drift of 10 ppm/°C. By choosing a precision reference with a low

output temperature coefficient, this error source can be minimized.

Table 7 suggests some of the references available from Analog

Devices, Inc., that are suitable for use with this range of current

output DACs.

Rev. D | Page 16 of 24

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